The efficacy of a therapeutic agent may be enhanced by improving its bioavailability via several means, one of which is PEGylation, a process of chemically linking polyethylene glycol (PEG) to the therapeutic agent of interest, with the resulting conjugate exhibiting an increased serum half-life. Additional advantages of the PEGylated products may also include lower immunogenicity, decreased dosing frequency, increased solubility, enhanced stability, and reduced renal clearance. Because the most common reactive sites on proteins (including peptides) for attaching PEG are the ε amino groups of lysine and the α amino group of the N-terminal residue, early methods of PEGylation resulted in modification of multiple sites, yielding not only monoPEGylated conjugates consisting of mixtures of positional isomers, such as PEGINTRONT™ (Grace et al., J. Biol. Chem. 2005; 280:6327) and PEGASYS® (Dhalluin et al., Bioconjugate Chem. 2005; 16:504), but also adducts comprising more than one PEG chain. Site-specific attachment of a single PEG to the α amino group of the N-terminal residue was reported to be the predominant product upon reacting PEG-aldehyde (PEG-ALD) at low pH with IFN-β1b (Basu et al., Bioconjugate Chem. 2006; 17:618) or IFN-β1a (Pepinsky et al., J. Pharmacol. Exp. Ther. 2001; 297:1059). Similar strategies were applied to prepare N-terminally linked PEG to G-CSF (Kinstler et al., Pharm. Res. 1996; 13:996) or type I soluble tumor necrosis factor receptor (Kerwin et al., Protein Sci. 2002; 11:1825). More recently, a solid-phase process for PEGylation of the N-terminus of recombinant interferon alpha-2a was reported (Lee et al., Bioconjug. Chem. Oct. 18, 2007, epub).
Site-directed PEGylation of a free cysteine residue introduced into a target protein has also been achieved with PEG-maleimide (PEG-MAL) for several recombinant constructs including IL-2 (Goodson and Katre, Biotechnology. 1990:8:343); IFN-α2 (Rosendahl et al., Bioconjugate Chem. 2005; 16:200); GM-CSF (Doherty et al., Bioconjugate Chem. 2005; 16:1291); scFv (Yang et al., Protein Eng. 2003; 16:761), and miniantibodies (Kubetzko et al., J. Biol. Chem; 2006; 201:35186). A popular approach for improving the therapeutic efficacy of an enzyme has been to prepare conjugates containing multiple PEG of small size, as known for methioninase (Yang et al., Cancer Res. 2004; 64:6673); L-methione γ-lyase (Takakura et al., Cancer Res. 2006:66:2807): arginine deaminase (Wang et al., Bioconjugate Chem. 2006; 17:1447); adenosine deaminase (Davis et al., Clin. Exp. Immunol. 1981; 46:649); L-asparaginase (Bendich et al., Clin. Exp. Immunol. 1982; 48:273); and liver catalase (Abuchowski et al., J. Biol. Chem. 1977; 252:3582).
PEGylations of bovine serum albumin (Abuchowski et al., J. Biol. Chem. 1977; 252:3578); hemoglobin (Manjula et al., Bioconjugate Chem. 2003; 14:464); visomant (Mosharraf et al., Int. J. Pharm. 2007; 336:215); small molecules such as inhibitors of integrin α4β1 (Pepinsky et al., J. Pharmacol. Exp. Ther. 2005; 312:742); lymphoma-targeting peptides (DeNardo et al., Clin. Cancer. Res. 2003; 9(Suppl.):3854s); anti-VEGF aptamer (Bunka and Stockley, Nat. Rev. Microbiol. 2006; 4:588) and oligodeoxynucleotides (Fisher et al., Drug Metab. Dispos. 2004; 32:983) have also been described. However, there exists a need for a general method of PEGylation that would produce exclusively a monoPEGylated conjugate composed of a single PEG linked site-specifically to a predetermined location of the candidate agent and retains the bioactivity of the unmodified counterpart.